The present invention relates to a golf head club, and more particularly, relates to a golf head club in which improvement in wall thickness is made on the back surface of a face part.
Most recent wood club heads are configured so that at least a face part is formed of a metallic material. The wall thickness of the face part must be increased so as to maintain a strength capable of withstanding a shock impacted by a ball. The increase in head size continues, and on the other hand, the rule specifies that the volume of a club head must be less than 460 cm3 plus an allowable error of 10 cm3. Therefore, the head volumes of most drivers are very close to the upper 460 cm3 limit. When the head size is increased, the sweet area expands, the peripheral weight distribution is emphasized, and the transverse and vertical moment of inertia increases, so that an error at the time of off-center hitting can be alleviated. However, if the head size is increased, and thereby the head weight is also increased, the swing balance increases, and the head speed drops, by which the carry may be decreased. To solve this problem, measures have been taken: the whole of the head may be formed of titanium or an alloy thereof (unless specially mentioned, hereinafter, “titanium or an alloy thereof” is referred to as “titanium”) having a small specific gravity and a high strength, or a composite head of carbon and titanium may be used.
Also, many types of highly resilient heads having not only large head size, but also a high restitution coefficient of the head, have been developed. From the year 2008, a highly resilient head having a restitution coefficient of 0.830 or more cannot be used in competition. So far, a thick-walled face material having an increased restitution coefficient has been used positively along with the increase in head size. However, even if a highly resilient head is used, when a ball is hit by a face part other than the sweet area, that is, at the time of off-center hitting, a spring effect cannot be anticipated, and the carry tends to decrease extremely.
Japanese Patent Application Publication No. 9-192273 describes a golf club head in which a center location including a sweet spot of a part forming a face is formed so as to have a thickness capable of holding a strength sufficient to withstand a shock given by a ball, and the thickness of a part around the circumference of the center location is made thinner than that of the center location, by which the whole face is provided with spring properties.
Japanese Patent Application Publication No. 2003-154040 also describes a golf club head provided with a thick-walled region in the center of a face member. This Publication describes a golf club head in which, taking an angle that an inclined line substantially perpendicular to the axis line of a shaft makes with a face line groove as θ, the thick-walled region has a substantially elliptical shape along an axis line inclining through the angle θ to the side opposite to the face line groove with the inclined line being an axis of symmetry.
Japanese Patent Application Publication No. 2008-132276 describes a golf club head provided with a face part including a central thick-walled part provided in a central region on the back surface of face, at least four ribs extending from the central thick-walled part to the peripheral edge of the face, and thin-walled parts formed between the adjacent ribs.
In the case in which the center of the face part is made thick, as described in Japanese Patent Application Publication No. 9-192273, when a ball is hit by a face part deviating from the sweet spot, the resilience performance of face part decreases greatly compared with the case in which a ball is hit by the sweet spot. Therefore, there arises a problem in that if the resilience performance in the sweet spot is kept low, the resilience performance in a face part other than the sweet spot decreases remarkably. Also, generally, as the face part is made thinner, the resilience performance of the face part tends to become higher and the strength of the face part tends to become weaker. Therefore, there arise problems in that it is difficult to maintain the strength of the face part while keeping the weight of the face part low, and that it is difficult to obtain resilience performance close to that of the sweet spot even when a ball is hit by a face part deviating from the sweet spot while keeping the resilience performance in the sweet spot low.